1. Field of the Invention
This invention relates to a method of modifying the profile of rigid or semi-rigid pipe formed of thermoplastic material, or rigid or semi-rigid, multi-walled plastic pipe and a method and apparatus for the manufacture of such pipe.
2. Description of the Prior Art
Rigid or semi-rigid plastic pipes have a variety of uses not least among which are uses as electrical conduit and drainage pipe which are frequently buried and may therefore be under considerable crushing pressures.
When plastic pipe was first utilized for such above identified uses, it tended to have a single, rather thick, smooth wall to withstand pressure.
Pipe having annular or helical corrugations in its wall was found to have advantages in withstanding pressure and the manufacture of such pipe was described and claimed in U.S. Pat. No. 3,981,663 issued to Manfred Lupke on Sep. 21, 1976. This patent is merely exemplary of a large number of patents relating to corrugated single-walled plastic tube and its production.
For various reasons including greater strength and, for smooth unfouled flow in drainage pipe, it became conventional to utilize double- walled corrugated pipe in which the outer wall was corrugated and the inner wall was smooth. Such pipe and a method and apparatus for its production are described in U.S. Pat. No. 4,226,580 issued to Gerd Lupke on Oct. 7, 1980. Again this patent is merely exemplary of the prior art.
More recently, the use of ribbed pipe has become popular for similar uses. Ribbed pipe comprises a single wall having annular or helical ribs upstanding therefrom. Such pipe may be made with considerable strength and for some purposes it may have advantages over double-walled, corrugated pipe. The disclosure of U.S. Pat. No. 4,712,993 issued Dec. 15, 1987 to Manfred Lupke is exemplary of ribbed pipe and one method of its manufacture.
The outer profile of both ribbed and corrugated pipe is ridged and, for some pipes, may cause appreciable inconvenience. For example, ribbed or corrugated pipe of large diameter having prominent ribs or corrugations may be difficult to drag over the ground due to the great frictional affect of the ribs or corrugations. It is known to the applicant to form corrugated pipe in a mold such as to initially mold corrugations with a somewhat flattened crest. Such flat topped corrugations may slightly mitigate any problems arising in stacking or moving tube. Nevertheless, the corrugations are still present and still may inhibit easy movement and stacking, and, indeed, the flat topped corrugations may even slightly reduce some of the advantages of corrugated pipe. Moreover, dirt and other detritus may accumulate in the troughs.
Thus, there appears to be problems of, firstly the provision of strength and, secondly, the easy maneuverability of large diameter pipe. Either problem alone may be mitigated by existing technology but, together, the solutions to one problem seem irreconcilable with solution to the other problem.
Generally, the problems are greater for ribbed pipe which was developed, inter alia, for the strength imparted by steep, deep annular or helical ribs. Any mobility difficulties are correspondingly greater. Moreover ribbed pipe does not have a second skin separated from the first skin by air gaps. Such skins as are present in corrugated pipe may have advantages in insulation and may also provide additional surety against puncture leading to pipe leakage where this is desirable. Such advantages of insulation and insurance against puncture have, hitherto, not been possible for ribbed pipe.
The present inventor has addressed these problems.